An ad-hoc network is a kind of self-configuring network that links using radio communication. The ad-hoc network is configured by plural nodes. Each of the nodes in the ad-hoc network transmits and receives packets using multi-hop communication. Multi-hop communication is a technique in which nodes each present outside the communication coverage of one another communicate through another node present in the communication coverage of each of the nodes.
As a technique using the ad-hoc network, according to one system, nodes capable of communicating by radio are incorporated in residential electric power meters; and thereby, the duties such as reading meters are executed through the ad-hoc network without any site visits by a worker. As to an ad-hoc network handling personal information such as the amount of electric power used by a household, ensure communication has to be executed from the viewpoints of confidentiality, prevention of falsification, etc.
According to conventional systems, ensure communication is achieved by encrypting packets transmitted and received among the nodes in the ad-hoc network. According to related techniques, an encryption key used to encrypt the packets is securely distributed (see, e.g., Japanese Laid-Open Patent Publication Nos. 2003-348072, 2010-98597, and 2007-88799). According to another technique, a node finds a gateway that is connected from the inside of the ad-hoc network (see, e.g., Japanese Laid-Open Patent Publication No. 2009-81854). According to another technique, a node obtains a bus generation number from another node that is a bus manager; and when the node becomes the bus manager, the node adds one to the obtained bus generation number and uses the resulting number (see, e.g., Japanese Laid-Open Patent Publication No. 2006-128876).
In an ad-hoc network, even when packets are encrypted, a proper packet transmitted within the ad-hoc network in the past may be captured by an attacker. The attacker can make an attack to cause the network to be congested by retransmitting the captured proper packet in the ad-hoc network (a replay attack). To ensure the communication quality as a network, the ad-hoc network has to be prepared to cope with a replay attack. For a conventional system, a technique is present according to which a node transmitting a packet stores into the packet, the transmission time of the packet; and the node receiving the packet compares the time of the node receiving the packet and the transmission time stored in the received packet, regards the comparison result as a result of execution of a replay attack if the times are far from one another, and discards the packet.
Nonetheless, according to the conventional techniques, the times of all the nodes in the ad-hoc network have to be synchronized. For example, the time synchronization for all the nodes in the ad-hoc network is implemented by regularly broadcasting a packet for the time synchronization, by a gateway that is a relaying device between the ad-hoc network and another network, or a specific node in the ad-hoc network.
Therefore, in an ad-hoc network executing meter reading, the broadcast packets for the time synchronization are transmitted and received in addition to the meter reading data packets for the meters. Therefore, a problem arises in that the communication load on the ad-hoc network increases.